Many social media applications, such as recommendation, virality prediction and marketing, make use of social graphs which indicate connections between users. However, these social graphs may not be explicitly specified or may be kept privately.
Meanwhile, advanced mobile devices, such as smartphones and wearable devices, are becoming an increasingly integrated part of people's lives, and a massive amount of user images are being shared everyday. These user-shared images are often publicly or easily accessible by others, for example, through the Internet.